Field Name

Size (bytes)

Description

Op

1

Operation Code: Specifies the type of message. A value of 1 indicates a request (BOOTREQUEST message) while a value of 2 is a reply (BOOTREPLY message).

HType

1

HLen

1

Hardware Address Length: Specifies how long hardware addresses are in this message. For Ethernet or other networks using IEEE 802 MAC addresses, the value is 6. This too is the same as the field with a similar name (HLN) in the ARP field format.

Hops

1

Hops: Set to 0 by a client before transmitting a request and used by BOOTP relay agents to control the forwarding of BOOTP messages.

XID

4

Transaction Identifier: A 32-bit identification field generated by the client, to allow it to match up the request with replies received from BOOTP servers.

Secs

2

Seconds: According to RFC 951, the client enters into this field the number of seconds “elapsed since [the] client started trying to boot”. This is supposed to provide information to BOOTP servers to help them decide which requests to respond to first.

Unfortunately, this definition was somewhat vague; it wasn't clear if this meant the amount of time since the machine was powered on, or since the first BOOTREQUEST message was sent. In addition, some devices incorrectly implemented this field. As a result, it is not always used.

Flags

2

CIAddr

4

Client IP Address: If the client has a current IP address that it plans to keep using, it puts it in this field. By filling in this field, the client is committing to responding to unicast IP datagrams sent to this address. Otherwise, it sets this field to all zero to tell the server it wants an address assigned. See the end of the detailed operation topic for important information on this field.

YIAddr

4

“Your” IP Address: The IP address that the server is assigning to the client. This may be different than the IP address currently used by the client. See the topic describing BOOTP operation in detail for an explanation of what happens in that case.

SIAddr

4

Server IP Address: The IP address of the BOOTP server sending a BOOTREPLY message.

GIAddr

4

Gateway IP Address: This field is used to route BOOTP messages when BOOTP relay agents facilitate the communication of BOOTP requests and replies between a client and a server on different subnets or networks. To understand the name, remember that the old TCP/IP term for “router” is “gateway”; BOOTP relay agents are typically routers.

Note that this field is set to 0 by the client and should be ignored by the client when processing a BOOTREPLY. It specifically does not represent the server giving the client the address of a default router address to be used for general IP routing purposes.

CHAddr

16

Client Hardware Address: The hardware (layer two) address of the client sending a BOOTREPLY. It is used both to look up a device's assigned IP address and also possibly in delivery of a reply message.

SName

64

Server Name: The server sending a BOOTREPLY may optionally put its name in this field. This can be a simple text “nickname” or a fully-qualified DNS domain name (such as “myserver.organization.org”).

Note that a client may specify a name in this field when it creates its request. If it does so, it is saying that it wants to get a reply only from the BOOTP server with this name. This may be done to ensure that the client is able to access a particular boot file stored on only one server

File

128

Boot Filename: Contains the full directory path and file name of a boot file that can be downloaded by the client to complete its bootstrapping process. The client may request a particular type of file by entering a text description here, or may leave the field blank and the server will supply the filename of the default file.

Vend

64

Vendor-Specific Area: Originally created to allow vendors to customize BOOTP to the needs of different types of hardware, this field is now also used to hold additional vendor-independent configuration information. See the next topic, on BOOTP vendor information extensions. It may be used by the client and/or the server.